Observation of the Kondo Effect in Multilayer Single-CrystallineVTe2 NanoplatesHongtao Liu,†,# Yunzhou Xue,‡,# Jin-An Shi,§ Roger A. Guzman,§ Panpan Zhang,† Zhang Zhou,†

Yangu He,†,⊥ Ce Bian,† Liangmei Wu,† Ruisong Ma,† Jiancui Chen,† Jiahao Yan,† Haitao Yang,†

Cheng-Min Shen,†,∥ Wu Zhou,§ Lihong Bao,*,†,∥ and Hong-Jun Gao†

†Institute of Physics & University of Chinese Academy of Sciences, Chinese Academy of Sciences, Beijing 100190, P.R. China‡College of Chemistry and Environmental Engineering, Shenzhen University, Shenzhen 518060, P.R. China§School of Physical Sciences and CAS Key Laboratory of Vacuum Physics, University of Chinese Academy of Sciences, Beijing100049, China∥Songshan Lake Materials Laboratory, Dongguan, Guangdong 523808, P.R. China⊥Department of Physics, Applied Physics, and Astronomy, Rensselaer Polytechnic Institute, 110 Eighth Street, Troy, New York12180, United States

*S Supporting Information

ABSTRACT: We report the chemical vapor deposition(CVD) growth, characterization, and low-temperature mag-netotransport of 1T phase multilayer single-crystalline VTe2nanoplates. The transport studies reveal that no sign ofintrinsic long-range ferromagnetism but localized magneticmoments exist in the individual multilayer metallic VTe2nanoplates. The localized moments give rise to the Kondoeffect, evidenced by logarithmical increment of resistivity withdecreasing temperature and negative magnetoresistance(NMR) regardless of the direction of magnetic field at temperatures below the resistivity minimum. The low-temperatureresistivity upturn is well described by the Hamann equation, and the NMR at different temperatures, a manifestation of themagnetization of the localized spins, is well fitted to a Brillouin function for S = 1/2. Density functional theory calculationsreveal that the localized magnetic moments mainly come from the interstitial vanadium ions in the VTe2 nanoplates. Our resultswill shed light on the study of magnetic properties, strong correlation, and many-body physics in two-dimensional metallictransition metal dichalcogenides.

KEYWORDS: VTe2, chemical vapor deposition, low-temperature transport, Kondo effect, DFT calculations

Recent discovery of intrinsic ferromagnetism in few-layertwo-dimensional (2D) van der Waals crystals, such asCrI3 and Cr2Ge2Te6,

1,2 has inspired active research inenriching the library of 2D magnetic materials.3−5 Theoreticalcalculations have predicted that monolayer vanadium-basedtransition metal dichalcogenides (TMD) with chemicalformula of VX2 (X = S, Se, Te) are intrinsicallyferromagnetic.6−10 Strong room-temperature ferromagnetismhas been observed in monolayer VSe2 grown on HOPG andMoS2 by molecular beam epitaxy.

11 However, the ferromag-netic order in VSe2 diminishes rapidly as the layer numberincreases, and bulk VSe2 is considered paramagnetic. Magneticproperties in bulk VSe2 are attributed to the interstitialvanadium ions.12−15 The interstitial vanadium ions providelocal magnetic moments due to localized 3d electrons, and thelocalized magnetic moments induced Kondo effect in bulkVSe2 has been observed.

16 A lattice of interstitial vanadiumions in V5S8 (or V0.25VS2) gives rise to antiferromagneticordering in bulk and weak ferromagnetism in ultrathin V5S8flakes.17,18 However, whether the local magnetic moments will

be formed by interstitial vanadium ions in VTe2 remainselusive.19

Dilute magnetic impurities in a metal will lead to the Kondoeffect.20 The Kondo effect is a many-body interaction betweenconduction electrons of nonmagnetic host and local momentof individual magnetic impurity through s-d exchangeinteraction, which leads to a resistivity minimum welldescribed by Δρ = −c ln T, where T is the temperature andc is a parameter depending on the host metal and the speciesand concentration of magnetic impurities.20,21 This expressionis only valid for temperatures well above the characteristicKondo temperature TK, as the Ruderman−Kittel−Kasuya−Yosida (RKKY) interaction between the dilute magneticimpurities through the conduction electrons can be neglectedat the temperature around and above TK.

20 The impurities are

Received: July 29, 2019Revised: October 26, 2019Published: November 8, 2019

Letter

pubs.acs.org/NanoLettCite This: Nano Lett. 2019, 19, 8572−8580

© 2019 American Chemical Society 8572 DOI: 10.1021/acs.nanolett.9b03100Nano Lett. 2019, 19, 8572−8580

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pubs.acs.org/NanoLetthttp://pubs.acs.org/action/showCitFormats?doi=10.1021/acs.nanolett.9b03100http://dx.doi.org/10.1021/acs.nanolett.9b03100

paramagnetic at the temperature above TK. However, for T ≪TK the spin of magnetic impurities will be compensatedforming a many-body singlet ground state, the Kondo singlet.Resistivity saturates toward T → 0 K due to the nonmagneticscattering of the Kondo singlet. Dilute magnetic impurities aretypically introduced by defects or magnetic dopants.22−27 Asmentioned above, the unpaired d-electron of vanadium ions,particularly the interstitial vanadium ions, in VX2 can give riseto localized moments or even magnetic ordering. Predictably, atrace amount of interstitial vanadium ions in VX2 will lead tothe Kondo effect.16,19 Quite recently single-crystalline VTe2nanoplates with thickness ranging from ∼90 to 8 nm have beengrown by chemical vapor deposition (CVD) and room-temperature ferromagnetism is observed.28 However, themagnetic properties are obtained in samples of VTe2nanoplates with various thicknesses and sizes. Magneticproperties of individual VTe2 nanoplates, particularly studiedby transport, and the origin of magnetic moments remainelusive.Here, we report the preparation, characterization, and low-

temperature magnetotransport of high-quality single-crystalline1T phase multilayer VTe2 nanoplates. The VTe2 nanoplatesare grown by a sublimed-salt-assisted atmospheric pressureCVD method and characterized by optical microscope (OM),atomic force microscopy (AFM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), scanning electronmicroscopy (SEM), and transmission electron microscopy(TEM). Low-temperature transport studies demonstrate thatall the multilayer VTe2 nanoplates are metallic and nosignature of long-range ferromagnetic order exists. Instead,localized magnetic moments are presented in the individual

multilayer VTe2 nanoplates, which lead to the Kondo effect.The Kondo effect is manifested by low-temperature upturn inresistivity and negative magnetoresistance (NMR) independ-ent of the direction of magnetic field. Scattering of conductionelectrons by the localized magnetic moments through isotropics-d exchange interaction leads to the resistivity minimum andNMR at low magnetic fields, both of which are analyzed withinthe framework of the Kondo model. Density functional theory(DFT) calculations demonstrate that interstitial vanadium ionsin VTe2 provide the localized moments.1T phase multilayer VTe2 nanoplates were synthesized by a

sublimed-salt-assisted atmospheric pressure CVD method.29

Tellurium lump and finely ground V2O5/NH4Cl (1:20, wt)powder were used as the tellurium source and vanadiumsource, respectively. VTe2 nanoplates were grown at thetemperature of 750 °C in 20 min followed by rapid coolingdown to room temperature with 300 sccm hydrogen/argon(1:9, v/v) as carrier gas in the whole process. Before growth,the furnace was first evacuated by a rotary pump to remove anyresidual air in the system and then filled with the carrier gas toatmospheric pressure. The evacuation process is of vitalimportance to the growth of VTe2 nanoplates. No VTe2nanoplates or VTe2 nanoplates with poor quality will growwithout the pre-evacuation process. Some VTe2 nanoplatesgrow vertically on the substrate, which makes the transfer ofVTe2 nanoplates to other substrates much easier without anycontamination.Figure 1a shows the atomic structure of 1T phase VTe2. It

crystallized in a trigonal layered structure in the space groupP3̅m1 (164) with lattice constants of a = b = 3.636 Å and c =6.51 Å.28 Each monolayer is composed of a layer of

Figure 1. VTe2 nanoplates grown by a sublimed-salt-assisted atmospheric pressure CVD method. (a) Atomic model of 1T phase VTe2. (b) Opticalimage of the hexagonal-shape VTe2 nanoplates grown on mica. The black line is a vertical standing VTe2 nanoplate. (c) AFM image and heightprofile of a VTe2 nanoplate with a thickness of 75.5 nm. (d) Room-temperature XRD pattern of the VTe2 nanoplates.

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Figure 2. TEM characterization of the single crystallinity of the as-grown 1T phase VTe2 nanoplates. (a) Low-magnification TEM image of a VTe2nanoplate and the corresponding EDS elemental mapping images of (b) vanadium (V) and (c) tellurium (Te) for the upper right corner of thenanoplate in (a). Inset in (a) is the SAED pattern of the nanoplate. (d) Low-magnification and (e) close-up HAADF-STEM images of the VTe2nanoplate viewed along the ⟨001⟩ zone axis. The Te atoms are the brighter columns, whereas the V columns show weak contrast. Green and reddots correspond to Te and V, respectively. (f) HAADF-STEM image acquired simultaneously during EELS mapping. EELS mapping of the (g) VL2,3 edge, (h) Te M4,5 edge, and (i) RGB colored map enhancing the contrast of the V and Te maps. V is red and Te is green.

Figure 3. Low-temperature transport properties of the multilayer VTe2 nanoplates. (a) Temperature-dependent resistivity (ρ−T) for the devicewith optical image showed in the inset. (b) Logarithmic low-temperature-dependent resistivity under different magnetic fields. Solid lines are fits tothe Kondo model. Relative magnetoresistance (MR) at different temperatures with the magnetic field (c) perpendicular (transverse MR) and (d)parallel (longitudinal MR) to the plane of the sample, respectively. (e,f) Transverse and longitudinal MR with low magnetic field fits (solid lines) toexpression MR (%) = aH2 + b at 1.9 and 20 K respectively.

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hexagonally arranged vanadium ions, sandwiched between twolayers of tellurium ions in octahedral coordination. The layersare held together via weak van der Waals forces. Figure 1bshows a typical optical image of the VTe2 nanoplates grown onmica. The as-grown multilayer VTe2 nanoplates are usually inhexagonal or semihexagonal shape, whereas for ultrathin VTe2nanoplates they are usually suborbicular (Figures 1b,c and S2−S6). Room-temperature XRD pattern (Figure 1d) of the VTe2nanoplates with various thicknesses and shapes agrees wellwith that of 1T phase VTe2,

28 suggesting that all the VTe2nanoplates share the same 1T phase crystalline structure. TheVTe2 nanoplates may segregate from tellurium liquid drops viaa vapor−liquid−solid growth mechanism, which is shown inFigure S2b. The single-crystalline structure of the 1T phaseVTe2 is further confirmed by TEM as shown in Figure 2.Energy dispersive spectrum (EDS) mappings of vanadium andtellurium show uniform distribution of the two elements(Figure 2b,c). The atomic ratio of tellurium and vanadium isclose to 2:1, as shown in Figure S4e. Selected-area electrondiffractions (SAED) show clear spots and are almost the sameorientation at different locations of the VTe2 nanoplate(Figures 2a (inset) and S5), which demonstrates the single-crystalline nature of the VTe2 nanoplate. Atomic-resolutionhigh-angle annular dark-field scanning TEM (HAADF-STEM)images in Figures 2d,e and S7 show the perfect atomicstructure of the as-grown VTe2 nanoplates. The 1T structure ofVTe2 is further supported by HAADF-STEM and electronenergy loss spectrometry (EELS). It can be seen in Figure 2f−ithat tellurium atoms (brighter columns) form a hexagonal ringwith the weaker contrast vanadium columns in the center ofthe ring. There are no vanadium vacancies, tellurium vacancies,and other defects in all the examined VTe2 nanoplates,demonstrating the high quality of the as-grown VTe2nanoplates.To investigate the low-temperature transport properties of

the as-grown VTe2 nanoplates, Hall bar devices were fabricatedby standard electron beam lithography (EBL) technique. Theinset in Figure 3a shows the optical image of a VTe2 nanoplatedevice with the nanoplate thickness of about 87 nm (FigureS10a). Temperature-dependent resistivity (ρ−T) in Figure 3ademonstrates the metallic properties of the VTe2 nanoplatewith a relatively low residual resistivity ratio (RRR = ρ300 K/ρ2K= 4.58), which is most likely due to the presence of a slightamount of interstitial vanadium ions.19,30 At high temperatures,the resistivity increases linearly with temperature due tophonon scattering. At low temperatures, the resistivity upturnarises and the resistivity minimum appears at about 8 K (Tm).When applying a strong magnetic field, the resistivity upturn issuppressed as seen in Figure 3b. Usually, resistivity upturncaused by the Kondo effect and weak localization effectdisappears under a strong magnetic field, whereas that causedby electron−electron (e−e) interactions cannot be affected bymagnetic field.31−33 To find the origin of the resistivity upturn,magnetoresistance (MR) was measured at different temper-atures. Figure 3c shows the transverse MR, which is measuredunder the magnetic field perpendicular to the electrical currentand the ab plane of VTe2 nanoplate. For T < Tm, VTe2 exhibitsnegative MR (NMR) at low fields. As shown in Figure 3c, themagnetoresistance becomes less steep for high magnetic fieldsand saturates in fields above about 5 T, exhibiting a crossoverfrom NMR to positive MR. The magnitude of NMR decreaseswith increasing temperature. When T > Tm, the NMR vanishesand positive MR gradually appears. When the magnetic fields

are parallel to the electrical current and ab plane of VTe2nanoplate, the longitudinal MR is still negative at lowtemperatures (Figure 3d). Figure 3e demonstrates that boththe transverse and longitudinal NMR are almost the same andthe NMR varies quadratically with increasing magnetic field atlow fields. When the contribution of positive quadraticordinary MR is subtracted in the transverse MR, the transverseand longitudinal NMR are almost exactly the same (FigureS9). However, the positive transverse and longitudinal MR arequite different (Figure 3f). The positive MR is suppressedwhen the magnetic field is parallel to the electrical current. Inother words, the NMR is isotropic and independent of thedirection of the magnetic field. However, NMR due to weaklocalization is anisotropic and vanishes when the magnetic fieldis parallel to the electrical current.34 As a consequence, theupturn in resistivity and NMR at low temperatures areattributed to the Kondo effect in which conducting electronsare scattered by localized magnetic moments through isotropics-d exchange interaction.The logarithmic upturn in resistivity at low temperatures can

be fitted within the framework of the Kondo effect. Theresistivity at temperatures ranging from 1.9 to 20 K can be wellfitted with the equation35,36

T qT pT

TT

lnTT

S S

( )

1 ln ( 1)

02 5

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where the first term is the residual resistivity, the second termrepresents the Fermi liquid contribution, the third termrepresents electron−phonon contribution, and the fourth termis the Kondo resistivity, which is described by the Hamannexpression, where, ρK0 is temperature-independent resistivity,TK is the Kondo temperature, and S is the spin of the magneticimpurities. From the fit, TK ∼ 6.2 ± 0.45 K and S ∼ 0.12 ±0.04 are obtained. As shown in Figure 3b, when the magneticfield is applied, broad peaks appear below TK, and theresistivity curve flattens out with increasing field. This reflectsthe splitting of the Kondo resonance by an applied magneticfield.33,37 The temperature dependent resistivity under differ-ent magnetic fields are fitted using eq 1 but with a modifiedHamann term37

H TTT

lnTT

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where B(x) is a Brillouin function for S = 1/2,

( ) ( )B x x x( ) coth cothS S S S S S2 12 2 12 12 12= −+ + , S is the magneticimpurity spin, g is the Lander factor and g = 2,34,38 μB is theBohr magneton, and kB is the Boltzmann constant. However,because the temperatures are still around or above TK, nosaturation of resistivity is observed in the absence of magneticfield.

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In spin-scattering models, the NMR is a function ofmagnetization of localized magnetic moments. It varies linearlywith the square of the effect ive magnet izat ion

MH( ) (0)(0)

2α− = − =ρρ

ρ ρρ

Δ − and is parabolically dependent

on the magnetic field S T H( ) 2− =ρρΔ at different temper-

atures.39,40 The coefficient S(T) is temperature-dependent andproportional to the square of the susceptibility of localized

spins ( )S T f( ) HT2χ∝ = .40,41 The value of S(T) can beextracted by fitting the NMR at different temperatures. Figure4a,b shows the temperature-dependent S(T) and S(T)−1/2

respectively. It can be seen that S(T) is inversely quadraticallydependent on temperature or S(T)−1/2 is proportional to

temperature, S T T T( ) ( )1/2 C∝ +− . This resembles the

Curie−Weiss law CT TC

χ = + , where χ is the magneticsusceptibility, C is the Curie constant, and TC is the Curietemperature. The fit in Figure 4b corresponds to a value of TC= 0.76 K. The positive TC suggests the antiferromagneticcoupling between the localized magnetic moments at T < TC.

40

However, the localized magnetic moments are paramagnetic atT > TC. The magnetic field dependence of square root of NMRis plotted in Figure 4c. It can be well fitted by a Brillouinfunction with S = 1/2. All the curves at different temperaturescollapse onto one single scaling curve in Figure 4d. The scalingcurve in Figure 4d can only be well fitted by a Brillouinfunction for S = 1/2

Ng SBg SH

k T0

1/2

BB

B

ρρ

μμΔ =

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(3)

The theoretical local magnetic moment of each magnetic

scatter center is g S S( 1)Bμ + = 1.73 μB, assuming quenchedorbital moment, which agrees well with the value ofintercalated vanadium ions in VX2 ranging from 1.4 to 2.49μB.

14,17,42

To reveal the origin of the localized magnetic moments,density functional theory (DFT) calculations were performed.Three types of defect were introduced to understand thestability and magnetic moment that can be brought withdefects, that is, tellurium single vacancy (VTe), vanadium singlevacancy (VV), and interstitial vanadium ion at interlayer space(VInter). The atomic configurations of three types of defect aredemonstrated in Figures 5a and S15, and the calculated

magnetic moments are demonstrated in Table 1. To evaluatethe defect formation energy, the chemical potential of vana-dium rich (V-rich) and tellurium rich (Te-rich) environmentare set as the energy per atom in the corresponding elementalphase of vanadium and tellurium, respectively. It can beinferred from Table 1 that in a V-rich environment, the VInterdefect has the lowest formation energy, which indicates thatwhen vanadium source is excessive, or locally excessive, theVInter defect is mostly likely to form thermodynamically. Thisagrees well with the fact that the V−X (X = S, Se, Te) systemstend to form self-intercalated compounds.43 Furthermore,

Figure 4. Low-field NMR characteristics at low temperatures.Temperature dependence of the parameter (a) S(T) and (b)[S(T)]−1/2 obtained from fits of the low-field NMR at differenttemperatures. Solid lines in (a,b) are inversely quadratic and linear fit,respectively. (−Δρ/ρ0)1/2 plotted as symbols versus (c) H and (d) H/T respectively. Solid lines in (c) are Brillouin function fits with S = 1/2. All the curves at different temperatures are superimposed togetherin (d) and can be well fitted by a Brillouin function with S = 1/2.Quadratic ordinary magnetoresistance obtained by fitting the highfield data have been subtracted.

Figure 5. Structure model and DFT simulations of vanadium ionsintercalated VTe2. (a) Atomic structure of vanadium ions intercalatedVTe2. (b) Total, V-3d, and Te-5p orbitals projected DOS ofvanadium ions intercalated VTe2. The Fermi level is set to zero.Positive and negative DOS correspond to spin-up and spin-downcomponents, respectively.

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VInter leads to the enhancement of magnetic moment of 0.73μB, which mainly provides the local moment leading to theKondo effect. The projected density of states (PDOS) of thedefect-free VTe2 and the three defect complexes are calculatedand demonstrated in Figures 5b and S15. The defect-free VTe2is spin polarized and ferromagnetic due to the unpaired 3d1

electron of each vanadium ion in VTe2, whereas thecontribution of the tellurium anion is small (Figure S15b). Itis consistent with the calculated magnetic properties of bulkVS2 and VSe2.

9,44,45 However, a previous report indicates thatthe ferromagnetism is suppressed by the charge-density-wavestate, and no intrinsic ferromagnetic ordering is observed inVTe2, even in the monolayer limit.

46 Although we do not havedirect evidence on the presence of charge-density-wave state inour VTe2 nanoplates, to clarify the competition between theferromagnetism and charge-density-wave in VTe2 furtherstudies are needed. When defects are introduced, the spinpolarization changes with respect to the defect configurations.For vanadium and tellurium vacancy, the spin polarizationweakens (Figure S15d,f). However, for vanadium ionsintercalated VTe2, the spin polarization is enhanced and isthe largest among all the samples. As shown in Figure 5b, theDOS of vanadium ions intercalated VTe2 demonstrates asignificant asymmetry between the spin-up and spin-downstates. As a consequence, we believe that the localizedmagnetic moments mainly originate from the interstitialvanadium ions. The interstitial V ions may come from thenonuniformly distributed vanadium precursor or fluctuationsin growth parameters such as growth temperature and carriergas, because of their low formation energy. It is also supportedby the fact that interstitial vanadium ions provide localizedmagnetic moments in V5S8 and VSe2.

16,42 We tried to find theinterstitial vanadium ions through HAADF-STEM image (Fig-ure S7), unfortunately, we did not find a clearly resolved imageto show the existence of the vanadium interstitials, which isprobably due to the fluctuation of distributed V interstitialsand the interstitials are in very small portion in the VTe2nanoplates.No change of the resistivity upturn and NMR in the VTe2

devices after stored for half a year suggests that the localizedmagnetic moments are not introduced by vanadium andtellurium vacancies (Figure S10). The degradation of VTe2during storage, which may introduce vanadium and telluriumvacancies, is confirmed by XPS (Figure S14). No anomalousHall effect is observed in all the VTe2 nanoplates with variousthicknesses in the whole range of temperature (Figure S12),which indicates that no intrinsic long-range ferromagneticorder occurs in the individual multilayer VTe2 nanoplates.Magnetization measurement also supports the transportresults. As-grown VTe2 nanoplates exhibit soft ferromagneticbehavior at low temperatures, whereas at high temperatures,

superparamagnetism or spin-glass-like state are observed(Figure S16), which probably originates from the inhomoge-neous interstitial vanadium ions or vanadium ion clusters. Theabsence of ferromagnetism in multilayer VTe2 nanoplates,particularly at room temperature, would be due to the largethickness or the existence of competing charge-density-waveground state11,46−51 or spin frustration analogous to VSe2.

52

The degradation will be exacerbated in ultrathin VTe2nanoplates and the e−e interactions (Altshuler−Aronov effect)will be enhanced. Meanwhile, previous studies on thethickness-dependent Kondo effect have shown that theKondo effect is partially suppressed with decreasing filmthickness.53 Consequently, a more pronounced Kondo effect,that is, large slope of the logarithmic temperature-dependentKondo resistivity, will be shown in thicker VTe2 nanoplates.Low-temperature transport studies of ultrathin VTe2 devicesshow that e−e interactions in thinner (34 nm) VTe2nanoplates emerges and leads to a resistivity upturn similarto the Kondo effect (Figure S13).32,54 In contrast to the Kondoeffect, the upturn in resistivity due to e−e interactions cannotbe suppressed by strong magnetic field. Furthermore, MR dueto e−e interactions is positive in ultrathin VTe2 nanoplates asshown in Figure S13b−f, whereas the Kondo effect leads toNMR at low magnetic fields and low temperatures. As aconsequence, in ultrathin VTe2 nanoplates, e−e interactionscompete with the Kondo effect, which will make the systemcomplicated. In order to obtain a relatively pure Kondo effect,multilayer thicker VTe2 nanoplates are much more preferable.Surface state also rules out multilayer thicker VTe2 nanoplates.The intercalated vanadium ions in VTe2 nanoplates are in traceamounts and randomly distributed, so they do not affect thecrystal structure of VTe2; there are no Kondo lattice forms inthis system.55 When more vanadium ions are intercalated inthe van der Waals gaps of VTe2, Kondo lattice or evenmagnetic ordering will be obtained.17,18,42

In conclusion, 1T phase multilayer VTe2 nanoplates havebeen successfully grown by atmospheric pressure CVD using asublimed salt as a growth promoter. The high-quality VTe2nanoplates are characterized by OM, AFM, XRD, XPS, SEM,and TEM. Low-temperature transport demonstrates thepresence of localized magnetic moments and the Kondo effectin the individual multilayer metallic VTe2 nanoplates, that is,logarithmic temperature dependence of resistivity upturnaccompanied with NMR at low magnetic fields at temperaturesbelow the resistivity minimum. The resistivity upturn can bewell fitted with the Hamann expression by which the Kondotemperature TK is obtained about 6.2 K. The NMR isindependent of the relative orientation of magnetic field andelectrical current, which suggests that conduction electrons arescattered by localized magnetic moments through isotropic s-dexchange interaction. DFT calculations demonstrate thatinterstitial vanadium ions are the main origin of the localizedmagnetic moments. The Kondo effect in ultrathin VTe2nanoplates is suppressed by the enhanced e−e interactionsor less interstitial vanadium ions. No signature of intrinsiclong-range ferromagnetic order is observed in the multilayerVTe2 nanoplates. Further studies still need to be carried out tostudy the formation, valence state, and concentration of theinterstitial vanadium ions in the VTe2 nanoplates. Controllableintroducing interstitial vanadium ions into VX2 and other 2DTMDs will enrich the material pool for spintronic applicationsand fundamental studies of magnetism at the 2D limit. Theresults presented here will lead to the study of the Kondo

Table 1. Calculated Formation Energy and MagneticMoment for Different Defects

formation energy(eV)

configuration V-rich Te-richmagnetic moment

(μB)moment difference

(μB)

perfect VTe2 1.30VTe −0.88 −0.40 0.21 −1.09VV 0.25 −0.72 1.30 0VInter −1.03 −0.05 2.03 0.73

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effect, strong correlation, or many-body physics in 2D TMDsand also shed light on the magnetic properties in VX2.Methods. CVD Growth of VTe2. VTe2 was grown in a two-

zone furnace by atmospheric pressure CVD. About 1 g oftellurium lump (Alfa Aesar, 99.999+%) was used as thetellurium source and placed in a quartz boat that was loadedupstream in the first zone of a two-zone furnace. 10 milligramsof the well-ground mixture of vanadium (V) oxide (V2O5, AlfaAesar, 99.99%) and ammonium chloride (NH4Cl, Alfa Aesar,99.999%) with a weight ratio of 1:20 was used as the vanadiumsource and placed in another quartz boat that was loadeddownstream in the second zone. The substrate (SiO2/Si ormica) was placed vertically about 1 cm downward from thevanadium source. Before ramping up the temperature, thefurnace was first evacuated for 5 min without carrier gas, andthen evacuated for another 5 min with 300 sscm hydrogen/argon (1:9, v/v). At last, the evacuation was stopped and thehydrogen/argon gas was filled in the furnace to atmosphericpressure. The temperature of tellurium and vanadium sourcewas set to 750 °C for 15 min and kept for 20 min. Then it wasrapidly cooled down to room temperature. Hydrogen/argon(300 sccm) was used as the carrier gas during the wholegrowth process.Characterization of VTe2. The as-grown VTe2 nanoplates

were characterized by optical microscope (Olympus BX51-SC30), scanning electron microscope (SEM, Hitachi S-4800,acceleration voltage of 10 kV, EDS 15 kV), atomic forcemicroscope (AFM, Digital Instruments Nanoscope IIIa)operated in tapping mode, transmission electron microscope(TEM, EM-2100F, JEOL, operating at 200 kV and equippedwith an EDS system), X-ray photoelectron spectroscopy (XPS,ESCALAB 250Xi spectrometer, Thermo Fisher Scientific), andX-ray diffraction (XRD, Bruker D2 phaser PHASERdiffractometer with Cu−Kα radiation, λ = 1.54184 Å)operating at 30 kV and 10 mA at room temperature.Aberration-corrected scanning transmission electron micros-copy (STEM) imaging and electron energy loss spectroscopy(EELS) mapping were performed using a Nion HERMES-100,operated at 100 kV. Because of partial overlapping of the V L-edge (513 eV) and Te M-edge (572 eV), the multiple linearleast-squares (MLLS) fitting method was used to extract thechemical map from as-acquired spectrum imaging.Device Fabrication and Transport Measurement. The as-

grown VTe2 nanoplates were transferred onto a precleanedSiO2/Si substrate using the face-to-face transfer method.

29 A300 nm thick poly(methyl methacrylate) (PMMA, molecularweights 950 K, 5% dilution of PMMA in anisole) film was spin-coated on it at a speed of 4000 rpm for 60 s. In order to avoiddegradation of the VTe2 nanoplate during the conventionalhigh-temperature baking, it was baked at 60 °C in a vacuumoven overnight. Hall bar devices were fabricated by a standardelectron beam lithography technique (Raith 150), followed byelectron-beam evaporation of 5/100 nm Ti/Au metal stacks ascontact electrodes. All transport measurements were per-formed in a physical property measurement system (PPMS,Quantum Design Inc.) using the resistivity option withalternating current (ac) drive mode. While performing theρ−T measurement, the cooling/warming rate was 2 K/min. Inorder to measure the longitudinal MR, a homemade rotatorrod was used. The direct current (dc) magnetizationmeasurements were carried out with a Quantum DesignMagnetic Property Measurement System MPMS SQUIDVSM. The VTe2 nanoplates were carefully transferred onto

Kapton tape for magnetization measurement to exclude anysignals from the substrate.

Density Functional Theory (DFT) Calculations. All DFTcalculations were performed with the Vienna ab initiosimulation package (VASP) using the Perdew−Burke−Ernzerhof (PBE) functional and projector augmented-wave(PAW) method.56−58 A supercell of 4 × 4 × 2 was constructedfor modeling different types of defects in which the interactionbetween defect and its image was larger than 1.3 nm. Thedispersive van der Waals interactions between different VTe2layers were considered using the DFT-D2 method ofGrimme.59,60 In each calculation, an energy cutoff of 520 eVwas adopted whereas a higher cutoff will have an energydifference of less than 0.01 eV. When performing the structureoptimizations, the system was regarded as converged when theforce per atom was less than 0.01 eV/Å.

■ ASSOCIATED CONTENT*S Supporting InformationThe Supporting Information is available free of charge athttps://pubs.acs.org/doi/10.1021/acs.nanolett.9b03100.

Additional figures regarding CVD growth, OM, SEM,SAED, TEM, transport, XPS, and DFT calculations ofVTe2 (PDF)

■ AUTHOR INFORMATIONCorresponding Author*E-mail: lhbao@iphy.ac.cn.ORCIDHongtao Liu: 0000-0003-0500-7204Lihong Bao: 0000-0002-2942-892XHong-Jun Gao: 0000-0002-6766-0623Author Contributions#H.L. and Y.X. contributed equally.NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSThis work was supported by National Key Research &Development Projects of China (Grants 2016YFA0202300,2018FYA0305800), National Natural Science Foundation ofChina (Grants 61674170, 61888102, 61904113), StrategicPriority Research Program of Chinese Academy of Sciences(CAS, Grants XDB30000000, XDB28000000), Youth Innova-tion Promotion Association of CAS (20150005), and Scienceand Technology Innovation Commission of Shenzhen(JCYJ20180305125616770). The authors would like to thankProf. Haifang Yang, Prof. Junjie Li, and Prof. Changzhi Gu inLaboratory of Microfabrication, Institute of Physics, CAS forthe support of device fabrication, and the STEM assistancefrom the Electron Microscopy Center of the ShenzhenUniversity.

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